Method of, and system for, reserving timeslots in a TDMA system

ABSTRACT

A method of, and system for, reserving timeslots in a TDMA system of a type in which a plurality of subscriber units (CPE) use the same radio channel for transmission during different timeslots to a base station and in which the base station administers the radio channel centrally enable subscriber units to transfer, if appropriate, a transmission request to the base station in the form of a PN (Pseudo-random noise) sequence (e.g., M-sequence, preferred gold, Katsami or orthogonal gold sequence). Transmission requests from different subscriber units can be identified at the base station using the PN sequence, reception time of the PN sequence and/or phase of the PN sequence.

The present invention relates to a method of, and system for, reservingtimeslots in a TDMA (time-division multiple access) system, in which aplurality of subscriber units use the same radio channel fortransmission during different timeslots, and a base station administersthe radio channel centrally.

Systems of the generic type and methods of the generic type are used inTDMA systems, in particular TDMA point-to-multipoint radio systems(PMP), to assign a subscriber unit ((CPE) customer premises equipment) achronologically limited range of a radio channel for transmission. InTDMA multipoint systems, a plurality of subscribers use the same radiochannel for transmission in chronological succession in the uplink, thatis to say when transmitting from the subscriber unit to the basestation. Since data is transmitted in a burst-like fashion, it isparticularly inefficient to transmit small data quantities, for examplea reservation request alone, as a preamble or a midamble is almostalways necessary for synchronization in the receiver when the data istransmitted. The actual transmitted information, that is to say forexample the reservation request, then takes up only a small portion ofthe total amount of data transmitted so that when a small number of bitsis transmitted in this way the spectral efficiency is particularly low.

The base station which administers the radio channel centrally assigns asubscriber a timeslot for transmission in the uplink, via acollision-free downlink when necessary.

As the time of a new request for a transmission timeslot cannotgenerally be predicted, it has already been proposed to perform a fixedassignment of timeslots so that each subscriber unit has thepossibility, at specific time intervals, of transmitting a reservationrequest to the base station in the uplink. However, a fixed assignmentof timeslots utilizes the transmission channel inefficiently as therewill be a large number of permanently allocated timeslots in which thesubscriber units do not output a reservation request.

In addition to the fixed assignment of timeslots, it has also beenproposed to interrogate the subscriber units at regular intervals. Thismethod is termed polling. If a subscriber unit is polled, a timeslot isallocated to it in which it can transmit data, that is to say bothreservation requests and subscriber data. However, in order to achieve ashort transmission delay, the subscriber units must be interrogated veryfrequently so that polling is also wasteful of transmission capacity.

It is also known to assign timeslots on the basis of what are referredto as random access timeslots. These are timeslots which are notassigned permanently to a subscriber unit. Instead, a plurality ofsubscribers can dispatch a reservation request in the timeslot whennecessary. However, it is disadvantageous that data is lost when thereis multiple access to the random access timeslot, that is to say whenthere is a collision. In the case of a collision, the subscriber unitsmust transmit again later. However, because the possibility ofcollisions repeatedly occurring is not excluded, long transmissiondelays are possible in a system using random access timeslots. In orderto limit these transmission delays it has been proposed, after acollision occurs, to increase the number of random access timeslotsand/or restrict the number of transmission-authorized subscriber units.However, this has the disadvantage that when there are smalltransmission delays such a method for resolving collisions leads towastage of the transmission bandwidth.

According to the present invention there is provided a method ofreserving timeslots in a TDMA system comprising a plurality ofsubscriber units (CPE) that use the same radio channel for transmissionduring different timeslots to a base station, and in which the basestation administers the radio channel centrally, which is characterizedby: if appropriate, the subscriber units transferring a transmissionrequest to the base station in the form of a PN (pseudo- random noise)sequence.

Here, it is, for example, possible to carry out ON/OFF signalling.Detection of the PN sequence of a subscriber unit by the base stationsindicates that data is present for transmission in the subscriber unit.If no sequence is detected, this is taken as indicating that thesubscriber unit does not request an uplink connection. Since the basestation is capable of identifying subscriber units from the PN sequence,this prevents collisions occurring. If a PN sequence is detected, it ispossible, for example in the next MAC (medium access control) frame, toallocate a timeslot for the transmission of data. More precisereservation data can then be transmitted with this using a “piggyback”method. It is also possible for a dedicated control timeslot to beallocated in the form of a short burst via which the only informationtransmitted then is a detailed reservation request. In all cases, it ispossible using the method of the invention to keep the time between thesubscriber unit dispatching the reservation request in the form of a PNsequence up to the availability of, for example, detailed filling levelinformation in a buffer of a subscriber unit, virtually constant, forexample of a length of two time periods of an MAC frame.

The use of PN sequences in data communication has already beensuccessfully implemented in CDMA systems. In CDMA systems, the actualtransmission data can be encoded on a subscriber-specific basis with aPN sequence in such a way that the transmission data in the base stationcan be unambiguously allocated to the subscriber unit. The presentinvention is to utilize the transmission of PN sequences within thescope of a TDMA system for identifying subscriber units during areservation request. In this way, transmission resources of the TDMAsystem are utilized efficiently and transmission delays are reduced.

Preferably, the PN sequence is an M-sequence. M-sequences areconveniently generated from fed-back shift registers with XOR logicoperation in the feedback branch. They can be used in differentreservation methods.

Advantageously, the PN sequence is a “preferred gold” sequence. This isconveniently obtained by performing an XOR logic operation on twoM-sequences which vary in phase with respect to one another.

In a further embodiment, the PN sequence is a “Katsami” sequence.Katsami sequences of different lengths and with different properties areknown such that subscriber units can unambiguously be distinguished fromone another.

In a further particularly advantageous embodiment, the PN sequence is anorthogonal gold sequence. Such sequences are “preferred gold” sequenceswhich have been lengthened by one element. It is also possible to carryout a selection in respect of the orthogonal gold sequences.

Preferably, the subscriber units transmit at least partially differentPN sequences at at least partially different times, and subscriber unitsare identified using the transmitted PN sequence and the reception timeof the PN sequence. In this way, it is possible, for a plurality ofsubscriber units to use the same PN sequence since the reception time ofthe PN sequence can be used to unambiguously identify the subscriberunit. Alternatively each subscriber unit can have a unique PN sequenceand subscriber units can be identified at the base station from the PNsequence alone.

In a further embodiment, the subscriber units transmit at leastpartially identical PN sequences at at least partially different times,and subscriber units are identified using the reception time of the PNsequence. Within the scope of this sequence-timing method it is, forexample, possible for all the subscriber units to use the sameM-sequence, but to broadcast them at staggered times. The identificationof subscriber units can then be carried out solely by means of thereception time of the PN sequencers.

In yet a further embodiment, the subscriber units transmit PN sequencesat different times and/or with different phases, and subscriber unitsare identified by using the reception time of the PN sequence and/or thephase. Here too, it is possible for all the subscriber units to use thesame PN sequence. Since different subscriber units use partiallydifferent times for transmitting the PN sequences it is possible toprovide the same phase for a plurality of subscriber units.

In a further advantageous embodiment, the subscriber units transmit PNsequences during the normal transmission operation, wherein the PNsequences lie below the noise level of the normal transmissionoperation, and the subscriber units are identified using the receptiontime. The signalling takes place, for example, using a long M-sequencebelow the noise level in parallel with the normal data transmission. Asa result of this the signal-to-noise ratio (SNR) of the normaltransmission is degraded, but, by means of suitable assumptions withrespect to the accessing subscriber units, it is possible to calculatethe level at which the noise power caused by the signalling can lie.

In a particularly preferred embodiment and in the case of thesequence-time method and in the case of the sequence-time-phase method,the transmission times lie within one timeslot. A timeslot is thereforedivided into a plurality of sub-timeslots with the result that the timeinformation used by the method to identify subscriber units can beacquired from the identification of the sub-timeslot.

Alternatively, the transmission times can lie within a plurality oftimeslots. In such a method the transmitted information can then beencoded so that even small SNR values permit a low error rate to be madeavailable by means of the coding gain.

Furthermore, in the case of the sequence-timing method and in the caseof the sequence-level method, a plurality of modulated sequences aretransmitted in succession. Coding is thus also possible within the scopeof these methods.

According to a second aspect of the invention there is provided a systemfor reserving timeslots in a TDMA system comprising: a plurality ofsubscriber units (CPE) that use the same radio channel for transmissionduring different timeslots to a base station, and in which the basestation administers the radio channel centrally, which is characterizedby, if appropriate, the subscriber units transferring a transmissionrequest to the base station using a PN (pseudo-random noise) sequence.

In this way, the aforementioned advantages of the method according tothe invention are implemented in a system for reserving timeslots.

Advantageously, the PN sequence is an M-sequence.

Preferably, the PN sequence is a “preferred gold” sequence.

In a further embodiment, the PN sequence is a “Katsami” sequence or anorthogonal gold sequence. The various PN sequences mentioned above canprovide different advantages depending on the method applied.

Furthermore, the system further comprises the subscriber unitstransmitting at least partially different PN sequences at at leastpartially different times, and identifying subscriber units using thetransmitted PN sequence and the reception time of the PN sequence.

Alternatively, the subscriber units transmit at least partiallyidentical PN sequences at at least partially different times, andsubscriber units are identified using the reception time of the PNsequence.

It is also envisaged that the subscriber units transmit PN sequences atdifferent times and/or with different phases, and for subscriber unitsto be identified using the reception time of the PN sequence and/or thephase.

In one further advantageous embodiment of the system, the subscriberunits transmit PN sequences during the normal transmission operation,wherein the PN sequences lie below the noise level of the normaltransmission operation, and subscriber units are identified using thereception time. To optimise the signal-to-noise ratio the PN sequenceare advantageously long sequences.

In the case of the sequence-time system and in the case of thesequence-time-phase system, the transmission times advantageously liewithin one timeslot. Such a timeslot is divided into a plurality ofsub-timeslots so that the time information used to identify subscriberunits can be acquired from the identification of the sub-timeslot.

Alternatively, in the case of the sequence-time system and in the caseof the sequence-time-phase system, the transmission times advantageouslylie within a plurality of timeslots. In this way coding of theinformation is possible.

In the case of the sequence-timing system and in the case of thesequence-level system, a plurality of modulated sequences are preferablytransmitted in succession. In these methods it is thus also possible toencode the information.

The present invention is based on the recognition that it is possible tosignal a transmission request to the base station in the form of apseudo-random noise (PN) sequence, and the base station being capable ofdetecting and unambiguously identifying the transmitting subscriber unitthat has transmitted the PN sequence. A method and a system are thusmade available which only require a small amount of transmissionresource for making reservation requests, whilst ensuring a shorttransmission delay. The more efficient use of the channel enables moredata to be transmitted, which reduces the transmission costs per bit.The short transmission delay improves the quality of the transmission,for example in the case of voice transmission.

Embodiments of the present invention will now be explained by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic view of an ISO OSI model for classifying thepresent invention within an entire communications system;

FIG. 2 shows an MAC frame with 64-QS(OG-3) sequences in order to clarifya sequence-time method or a sequence-time system according to thepresent invention;

FIG. 3 shows an MAC frame with a 127-M-sequence in order to clarify asequence-timing method or a sequence-timing system according to thepresent invention;

FIG. 4 shows an MAC frame with a 255-M-sequence in order to clarify asequence-time-phase method or a sequence-time-phase system according tothe present invention;

FIG. 5 shows an MAC frame with M-sequences in order to clarify asequence-level method or a sequence-level system according to thepresent invention;

FIG. 6 shows level diagrams in order to clarify a signal-level method ora signal-level system according to the present invention, and

FIG. 7 shows a diagram representing error probabilities as a function ofthe signal-to-noise ratio for binary signals.

In order that the invention can be better understood, the basicproperties of various PN (pseudo-random noise) sequences will firstly begiven in the following table. Here, in each case the sequence length,the number of available sequences, the autocorrelation and thepeak-cross-correlation are given for the individual sequence types.Quasi-synchronous timing of the reception sequences in the receiver is aprecondition for the indication of autocorrelation and ofcross-correlation for the QS(OG-3) sequences, that is to say an offsetwithin ±1.5 symbol for QS(OG-3) and ±0.5 symbol for QS(OG-1). TheM-sequences are obtained from fed-back shift registers with XOR logicoperation in the feedback branch. The “preferred gold” sequences areobtained by means of an XOR logic operation performed on two M-sequenceswhich vary in phase with respect to one another. QS(OG-1) sequences areorthogonal gold sequences (“preferred gold” sequences which have beenlengthened by one element). By means of selection, QS(OG-3) sequencesare obtained from the QS(OG-1) sequences.

Peak-cross- Sequence Length Number Autocorrelation correlationM-sequence 31 6   31/−1  11 M-sequence 63 6   63/−1  23 M-sequence 12718   127/−1  41 M-sequence 255 16   255/−1  95 M-sequence 511 48  511/−1  113 Preferred gold 31 33 31/9  9 (29%) Preferred gold 63 6563/17 17 (27%) Preferred gold 127 129 127/17  17 (13%) Preferred gold255 257 255/31  31 (12%) Preferred gold 511 513 511/33  33 (6%)Preferred gold 1023 1023 1023/65  65 (6%) Katsami sequence 63 8 63/9  9(14%) Katsami sequence 255 16 255/17  17 (7%) Katsami sequence 1023 321023/33  33 (3%) 4-QS(OG-1) 4 4 4 0 8-QS(OG-1) 8 8 8 0 16-QS(OG-1) 16 1616 0 32-QS(OG-1) 32 32 32 0 32-QS(OG-3) 32 8 32 0 64-QS(OG-3) 64 16 64 0128-QS(OG-3) 128 32 128 0 256-QS(OG-3) 256 64 256 0 512-QS(OG-3) 512 128512 0 1024-QS(OG-3) 1024 256 1024 0The different methods that will be described below are subject toperipheral conditions which have been assumed by way of example to be asfollows:

active CPEs in one sector: maximum of 256 symbol rate (uplink): 12.6M-symbols/s MAC frame length: 1 ms (=12600 symbols) SNR in the receiver:approx. 5 dB

In addition, it has been assumed that for the exemplary description ofthe various methods eight simultaneous reservation requests have beenmade.

FIG. 1 shows an overview of an ISO OSI model in order to be able toclassify the present invention within the entire system. The ISO OSImodel comprises a data link layer 10 and a physical layer 12. Thephysical layer 12 is divided into two layers, specifically a physicallayer convergence protocol (PLCP) 14 and into a physical mediumdependent (PND) layer 16. The data link layer 10 is also divided intotwo layers, specifically into a logic link control 18 (LLC) and into alayer for medium access control 20 (medium access control layer; MAClayer). The methods and systems described within the scope of thepresent invention are to be assigned to the MAC layer 20 within theentire system illustrated in FIG. 1.

The sequence of a plurality of MAC frames is represented in tabular formbelow:

12600 symbols 12600 symbols 12600 symbols 12600 symbols MAC frame N MACframe N + MAC frame N + MAC frame N + 1 2 3 1 ms 1 ms 1 ms 1 ms

FIG. 2 shows a MAC frame with 64-QS(OG-3) sequences in order toillustrate a sequence-time method or a sequence-time system according tothe present invention. Examples of sequences which can be used withinthe scope of the sequence-time method and their properties and theireffect on the transmission are represented in the following table.

Timeslot Computational Sequence (overall work per MAC Sequence numberlength) SNR frame 1024-QS(OG-3) 256 1030 35 dB 256 * 1024 * 5 512-QS(OG-3) 128 1033 32 dB  2 * 128 * 512 * 5  256-QS(OG-3) 64 1039 29dB  4 * 64 * 256 * 5  128-QS(OG-3) 32 1051 26 dB  8 * 32 * 128 * 5 64-QS(OG-3) 16 1075 23 dB  16 * 16 * 64 * 5  32-QS(OG-3) 8 1123 20 dB 32 * 8 * 32 * 5  16-QS(OG-3) 4 1219 17 dB  64 * 4 * 16 * 5   8-QS(OG-3)2 1411 14 dB 128 * 2 * 8 * 5

FIG. 2 illustrates a MAC frame 210 which is divided into a plurality oftimeslots 212, 214, 216, . . . , 218. The timeslots 214, 216, . . . ,218, of which three are illustrated in FIG. 2, are used for “normal”data transmission. The timeslot 212 makes available 256 subscriber unitsCPE 0, CPE 1, . . . , CPE 255 for dispatching reservation requests. Forthis purpose, the timeslot 212 is divided into 16 sub-timeslots 220,222, 224, . . . , 226, of which four are represented in FIG. 2. Each ofthe sub-timeslots 220, 222, 224, . . . , 226 has a length 228 of 16symbols. Within an individual sub-timeslot, the subscriber units 16 areassigned different codes C 0, C 1, C 2, . . . C 15. For example, thesubscriber units CPE 0, CPE 16, CPE 32, . . . , CPE 240 are assigned thecode C 0. The subscriber units are, however, distinguishable from oneanother when the code C 0 is received by the base station because eachof the subscriber units CPE 0, CPE 16, CPE 32, . . . , CPE 240 isassigned to a different sub-timeslot 220, 222, 224, . . . , 226. Anothercode, for example the code C 11 is assigned to the other subscriberunits, for example the subscriber units CPE 11, CPE 27, CPE 43, . . . ,CPE 251.

The high SNR values given in the table above result from the favourablecross-correlation function of the respective sequences.

In contrast to the illustration in FIG. 2, it would, for example, alsobe possible to operate with a 256-QS(OG-1) code with in each case onesequence for the 256 subscriber units. The timeslot length would then bea total of only 256 symbols. However, with a reception inaccuracy of ±1symbol severe SNR degradation is then possible under unfavourableconditions. To remedy this, the reception inaccuracy could then bereduced to ±0.5 symbol.

FIG. 3 shows an MAC frame with a 127-M-sequence in order to illustrate asequence-timing method or a sequence-timing system according to thepresent invention. Here, all the subscriber units use the sameM-sequence. The identification of the respective subscriber unit iscarried out in the receiver using the reception time.

In the following table, possible M-sequences to be used, and theirproperties and effects on the system, are given. In the SNR calculationit has been assumed that the autocorrelation function of the sequenceused is always −1 beyond the maximum. This applies to periodicallypropagated M-sequences. However, as the M-sequences are not periodicallypropagated here, when there is overlap of a plurality of sequences asmaller SNR is obtained. In order to counteract this, it is possible touse more suitable sequences.

Timeslot Computational Sequence (overall work per MAC Sequence numberlength) SNR frame 1023-M-sequence 1 1023 + 3 * 255 21.5 dB 256 * 1023 *5  511-M-sequence 1  511 + 3 * 255 18.5 dB 256 * 511 * 5  255-M-sequence1  255 + 3 * 255 15.5 dB 256 * 255 * 5  127-M-sequence 1  127 + 3 * 25512.5 dB 256 * 127 * 5  63-M-sequence 1  63 + 3 * 255  9.5 dB 256 * 63 *5

The MAC frame 310 is divided into a plurality of timeslots 312, 314,316, . . . , 318. Here, the timeslots 314, 316, . . . , 318, of whichthree are illustrated, are provided for the normal data transmission.The timeslot 312 is used to transfer reservation requests from 255subscriber units CPE 0, CPE 1, CPE 2, . . . , CPE 255. The sequence usedis a 127-M-sequence so that the timeslot 312 has a total length 328 of892 symbols.

FIG. 4 shows an MAC frame with a 255-M-sequence in order to explain asequence-time-phase method or a sequence-time-phase system according tothe present invention. M-sequences of different phases are transmittedby the various subscriber units. Different times are also used fordispatching the sequences so that this also serves as a distinguishingcriterion of the subscriber units.

First, the use of possible sequences will be given by way of example inthe following table, their properties and their effects on the entiresystem also being shown.

Timeslot Computational Sequence (overall work per MAC Sequence numberlength) SNR frame 1023-M-sequence 341 1027 21.5 dB 341 * 1023 * 5 (341CPEs)  511-M-sequence 170 1031 18.5 dB 340 * 511 * 5 (340 CPEs) 255-M-sequence  85 785 15.5 dB 255 * 255 * 5 (255 CPEs)  127-M-sequence 42 783 12.5 dB 252 * 127 * 5 (252 CPEs)  63-M-sequence  21 795  9.5 dB252 * 63 * 5 (252 CPEs)

If the reception time can fluctuate by more than ±1 symbol, the numberof phases which can be used is reduced so that fewer subscriber unitscan transmit within one timeslot. If there is uncertainty of ±4 symbols,only 28 subscriber units can then be used in a timeslot of length 263symbols.

The MAC frame 410 illustrated in FIG. 4 is divided into a plurality oftimeslots 412, 414, 416, . . . , 418. The timeslots 414, 416, . . . ,418, of which three are represented by way of example, are used for thenormal data transmission. The timeslot 412 is used to dispatchreservation requests. The timeslot 412 is divided into threesub-timeslots 420, 422, 424, 255 subscriber units CPE 0, CPE 1, CPE 2, .. . , CPE 254 are accommodated in the three sub-timeslots 420, 422, 424.The subscriber units which are represented in a same row in FIG. 4, thatis to say for example the subscriber units CPE 2, CPE 87 and CPE 172,use the same phase P6. Subscriber units which are represented indifferent columns in FIG. 4 use different phases, only every third phasebeing used owing to the uncertainty of the reception time of ±1 symbol.

The method illustrated in FIG. 4 is of interest for collision-freereservation. Given 85 active subscriber units, only one timeslot of thelength 255 (plus the guard time) is necessary with a 255-M-sequence.Given 8 simultaneous access operations, the SNR is then still 15.5 dBwith a detection error rate which is less than 2˜10⁻⁵. Given foursimultaneous access operations, the detection error rate drops to 10⁻¹⁰.

FIG. 5 shows an MAC frame with M-sequences in order to explain asequence-level method or a sequence-level system according to theinvention. Signalling is preferably carried out in parallel to thenormal data transmission using a long M-sequence that is below the noiselevel. The identification of the subscriber unit is carried out on thebasis of the time of the access. So that each subscriber unit can accessonce within an MAC frame (12600 symbols), the interval between twosequences must be less than or equal to 49.

Exemplary sequences, their properties and their effects on the entiresystem are illustrated in the following table.

Computational Sequence SNR SNR work per Sequence overlap signallingdegradation MAC frame 4095-M-sequence 84 13 dB 0.5 dB 256 * 4095 * 54095-M-sequence 84  7 dB 0.1 dB 256 * 4095 * 5 2047-M-sequence 42 10 dB0.5 dB 256 * 2047 * 5 1023-M-sequence 21  7 dB 0.5 dB 256 * 1023 * 5 511-M-sequence 11  4 dB 0.5 dB 256 * 511 * 5

A MAC frame 510 is shown in FIG. 2 which is divided into a plurality oftimeslots 512, 514, 516, 518, . . . , 520, five timeslots beingrepresented by way of example. Normal data transmission takes placeduring the timeslots 512, 514, 516, 518, . . . , 520, in which case, forexample, the timeslot 512 is assigned to the subscriber unit CPE X1 andthe timeslot 516 is assigned to the subscriber unit CPE X3. In parallelwith the normal data transmission, reservation requests are dispatchedin the form of access sequences 522, 524, 526, 528, 530, of which fiveare represented by way of example in FIG. 5. Each of the accesssequences has a length of 2047 symbols in the example.

In FIG. 6, signal levels are shown which occur in conjunction with anexemplary method using a 2047-M-sequence, the left-hand side of theillustration showing signal levels before despreading and the right-handside showing signal levels after despreading. The level 610 shows anaccess signal level for a subscriber unit. Level 612 corresponds to 8subscriber units. The level 612 is still 9 dB below the normal noiselevel 614, it being assumed for the transmission burst that it has, withrespect to the normal noise level 614, a level 616 with an SNR of 5 dB.After despreading, in which a despreading gain of 33 dB is assumed, anSNR of 10 dB is present with respect to the level 618 of the accesssignalling.

During the calculation of the SNR degradation, the most unfavourablecombination of subscriber units in the access operations was assumed,that is to say it was assumed that there were successive accessoperations. However, as in all the exemplary embodiments describedabove, a maximum of 8 access operations is assumed during an MAC frame.For an SNR degradation of 0.5 dB, the noise power caused by signallingmust be 9 dB below the normal noise level.

It is to be noted that within the scope of the signal-level method andthe signal-level system it is possible to achieve a small detectionerror probability, preferably with long sequences.

The degradation and necessary countermeasures are illustrated in a tablebelow for different paths for multi-path propagation for the methodsdescribed above.

Strong Strong long-range Method short-range echoes echoes Weak echoesSequence-time High high small method Sequence-timing medium-sized/medium-sized small method relatively large intervals Sequence-time-medium-sized medium-sized small phase method amount/relatively fewphases Sequence-level Small small small method

With the exception of the sequence-level method, all the methods reactto strong echoes in a sensitive way. With adaptive receivers, partial orcomplete compensation of multi-path propagation is possible with a goodSNR.

Furthermore, it should also be noted that given a known phase shift oramplitude change between the transmitter and receiver, for example, as aresult of repeated transmission, the transmitter can modulate the phaseor the amplitude with information. It is then also possible to userelatively high modulation types (QPSK, N-PSK, N-QAM, etc.). As aresult, additional information can be transmitted or the transmissioncan be protected by coding.

FIG. 7 shows a diagram representing error probabilities as a function ofthe signal-to-noise ratio for binary signals. Given an identicalprobability of occurrence of a reservation request or of the absence ofa reservation request, the probability of error detection is obtained inaccordance with FIG. 7. For a high detection probability, large SNRvalues are necessary, for example 13 dB for 10⁻³ detection errors. Thesevalues apply to ON/OFF signalling, as is represented by curve ‘a’. Ifantipodal levels are used for the transmission of a bit, an error curvewhich is better by 6 dB is obtained, as is illustrated in FIG. 7 by thecurve designated by ‘b’. However, in order to realize this, referencevalues (amplitude/phase) must be present.

The above description of the exemplary embodiments according to thepresent invention serves only for illustrative purposes and not for thepurpose of restricting the invention. Various changes and modificationsare possible within the framework of the invention without departingfrom the scope of the invention or its equivalents.

1. A method of reserving timeslots in a time division multiple accesssystem having a plurality of subscriber units that use a same radiochannel for transmission during different timeslots to a base station,and in which the base station administers the radio channel centrally,comprising the steps of: transferring transmission requests by thesubscriber units, in timeslots assigned by the base station, to the basestation in a form of pseudo-random noise (PN) sequences; transmittingthe PN sequences by the subscriber units at different times; andidentifying the subscriber units using reception times of the PNsequences.
 2. The method according to claim 1, in which each PN sequenceis an M-sequence.
 3. The method according to claim 1, in which each PNsequence is a preferred gold sequence.
 4. The method according to claim1, in which each PN sequence is a Katsami sequence.
 5. The methodaccording to claim 1, in which each PN sequence is an orthogonal goldsequence.
 6. The method according to claim 1, in which the PN sequencesare at least partially different.
 7. The method according to claim 1, inwhich the PN sequences are at least partially identical.
 8. The methodaccording to claim 1, and further comprising the steps of: thesubscriber units transmitting the PN sequences with different phases,and identifying the subscriber units using a phase of each PN sequence.9. The method according to claim 8, in which for the sequence-time stepsand for the sequence-time-phase steps, the transmission times lie withinone timeslot.
 10. The method according to claim 8, in which for thesequence-time steps and for the sequence-time-phase steps, thetransmission times lie within a plurality of the timeslots.
 11. Themethod according to claim 1, and further comprising the steps of: thesubscriber units transmitting the PN sequences during normaltransmission operation, wherein the PN sequences lie below a noise levelof the normal transmission operation.
 12. The method according to claim11, in which for the sequence-time steps and for the sequence-levelsteps, a plurality of modulated sequences is transmitted in succession.13. A subscriber unit for use in a communication system adapted tooperate in accordance with the method of claim
 1. 14. A base station foruse in a communication system adapted to operate in accordance with themethod of claim
 1. 15. A system for reserving timeslots in a timedivision multiple access system having a plurality of subscriber unitsthat use a same radio channel for transmission during differenttimeslots to a base station, and in which the base station administersthe radio channel centrally, comprising: the subscriber units beingoperative to transfer transmission requests, in timeslots assigned bythe base station, to the base station in a form of pseudo-random noise(PN) sequences; the subscriber units being further operative to transmitthe PN sequences at different times; and the base station beingoperative to identify the subscriber units using reception times of thePN sequences.
 16. The system according to claim 15, in which each PNsequence is an M-sequence.
 17. The system according to claim 15, inwhich each PN sequence is a preferred gold sequence.
 18. The systemaccording to claim 15, in which each PN sequence is a Katsami sequence.19. The system according to claim 15, in which each PN sequence is anorthogonal gold sequence.
 20. The system according to claim 15, in whichthe PN sequences are at least partially different.
 21. The systemaccording to claim 15, in which the PN sequences are at least partiallyidentical.
 22. The system according to claim 15, wherein the subscriberunits transmit the PN sequences with different phases, and identify thesubscriber units using a phase of each PN sequence.
 23. The systemaccording to claim 22, in which for the sequence-time systems and forthe sequence-time-phase system, the transmission times lie within onetimeslot.
 24. The system according to claim 22, in which for thesequence-time system and for the sequence-time-phase system, thetransmission times lie within a plurality of the timeslots.
 25. Thesystem according to claim 15, wherein the subscriber units transmit thePN sequences during normal transmission operation, and wherein the PNsequences lie below a noise level of the normal transmission operation.26. The system according to claim 25, in which for the sequence-timesystem and for the sequence-level system, a plurality of modulatedsequences is transmitted in succession.